The invention relates to a device for optically scanning a surface, which device comprises a radiation source for supplying a radiation beam, an objective lens mounted in an objective system for focusing the radiation beam on the surface and drive means for displacing the objective system over a long stroke crossing a multiplicity of scan-resolvable locations.
A device of this type is suitable, inter alia for reading disc-shaped optical record carriers in which the information is arranged in concentrical or spiral tracks. A single track on the record carrier can be scanned by rotating the record carrier. To scan other tracks, the objective lens must be moved radially across the tracks. For a short access time to the stored information, it must be possible to move the objective lens rapidly across the tracks.
A device as described in the opening paragraph is known from U.S. Pat. No. 4,363,116. In this known device the objective lens is secured in a carriage which can be displaced over a given stroke in the radial direction with respect to an optical record carrier to be scanned. The carriage is provided with a focus actuator for moving the objective lens in a direction perpendicular to the plane of the record carrier in order to keep the radiation beam focused in the plane of the tracks. The carriage is driven by a carriage actuator with which the carriage and hence the objective lens can be moved radially with respect to the record carrier. The carriage is also provided with an adjustable element in the form of a pivotal mirror for radially moving the focal spot in order to track small radial excursions of the track to be scanned, which excursions are too fast to be tracked by the carriage actuator. A non-movable part of the device includes an optical system accommodating a radiation source and a detection unit. The optical system supplies a collimated radiation beam which is passed to the carriage in the radial direction parallel to the plane of the record carrier. The pivotal mirror in the carriage, at a nominal angle of 45 .degree. to the radiation beam, deflects the radiation beam upwards to the objective lens. The mirror always receives the radiation beam, irrespective of the position of the carriage in said stroke.
A drawback of the known device is the relatively large mass of the carriage. Consequently, it is not possible to move the focal spot rapidly to an arbitrary radial position on the record carrier, in other words to realise a short access time to the information.